decouples depth from difficulty, and renames depth to ID

for future impl of branches

src/map_builders/voronoi_spawning.rs

@@ -42,7 +42,7 @@ impl VoronoiSpawning {
 
         // Spawn the entities
         for area in noise_areas.iter() {
-            spawner::spawn_region(&build_data.map, rng, area.1, build_data.map.depth, &mut build_data.spawn_list);
+            spawner::spawn_region(&build_data.map, rng, area.1, &mut build_data.spawn_list);
         }
     }
 }